In order to meet increased traffic demands and increased data rate requirements, heterogeneous cellular communications systems are being developed and deployed. The radio access network of a conventional cellular communications network is flat, i.e., has a single layer of base stations. In contrast, the radio access network of a heterogeneous cellular communications system includes multiple layers of base stations, or radio access nodes, which typically operate at different average power levels. For instance, there may be a macro cell layer formed by a number of macro cell, or high power, base stations and a small cell layer formed by a number of small cell, or low power, base stations. Heterogeneous cellular communications systems create new interference scenarios that did not occur in the conventional cellular communications system.
In heterogeneous cellular communications systems, one of the mechanisms for interference avoidance and coordination among small cells is use of a small cell on/off feature. According to this feature, a small cell may be turned on and off, where the “on” and “off” periods may depend on criteria or an application.
In semi-static small cell on/off, the criteria for causing the small cell to transition on/off may include traffic load on the small cell, wireless device arrival to the small cell or departure from the small cell, etc. On the other hand, in dynamic small cell on/off, a small cell can be caused to turn on and off dynamically on a subframe level. The criteria in this case can be packet arrival/completion or interference coordination and avoidance (i.e., to reduce interference towards other nodes or wireless devices). Thus, the small cell turns off at the subframe boundary (or at the end of a current subframe) when the transmission of a packet is completed, and turns on at the next subframe boundary where a packet arrives. In addition, another purpose of using small cell on/off includes energy savings because, for example, the small cell is not “on” constantly.
There are three main operational modes of small cell on/off including handover, Secondary Cell (SCell) only, and serving cell operational modes. In the handover operational mode, a wireless device (e.g., a User Equipment (UE) in Third Generation Partnership Project (3GPP)) in CONNECTED mode is always attached to a cell in a cellular communications system (i.e., a network). Due to, for example, increased traffic demand, the cellular communications system may decide to offload the wireless device by handover to a small cell. If the small cell is “off,” then the small cell wakes up (i.e., turns “on”) to serve the wireless device. The handover time in this case depends on a backhaul delay and the handover execution time. After completion of the transmission and/or reception of data, the wireless device changes to IDLE mode or is handed over to another cell, and the small cell can be turned “off.”
The SCell only mode of operation applies when a Carrier Aggregation (CA) scheme is used. CA schemes are schemes that allow a wireless device to transmit and/or receive on multiple Component Carriers (CCs). One CC is the Primary Component Carrier (PCC), where the Primary Cell (PCell) is on the PCC. A wireless device may also be configured with one or more SCells on one or more Secondary Component Carriers (SCCs).
In the SCell only operational mode of small cell on/off, a CA (also referred to as multi-carrier) capable wireless device is connected to a PCell, and the cellular communications system configures a SCell for the wireless device that can be turned on or off. If the cellular communications system decides to offload the wireless device traffic to a SCell, then the SCell is turned on. The cellular communications system may also configure more than one SCell for the wireless device, e.g., two SCells according to an on/off scheme. The PCell and SCell(s) may belong to the same frequency band or different frequency bands.
In the serving cell operational mode for small cell on/off, a small cell can be either on or off when a wireless device is connected to that small cell. The procedures for Radio Resource Management (RRM), Radio Link Monitoring (RLM), and Channel State Information (CSI) measurements must be designed for this mode.
An on/off discovery signal feature may be used in the cellular communications system. According to this feature, a small cell may be turned on and off where the “on” and “off” occasions occur periodically. During an “on” period, a network node (e.g., a base station) transmits one or more reference signals that enable a wireless device to perform measurements on the small cell controlled by the network node. During an “off” period, a network node (e.g., a base station) periodically (e.g., every 40 ms, 80 ms or 120 ms) transmits a discovery signal that enables a wireless device to discover the small cell. A discovery signal, as referred to herein, is any kind of periodic reference signal or pilot signal known to the wireless device. Examples of discovery signals include Cell-Specific Reference Signals (CRSs), CSI Reference Signals (CSI-RSs), Primary Synchronization Signals (PSSs), Secondary Synchronization Signals (SSSs), Positioning Reference Signals (PRSs), etc.
A transmission of on/off discovery signal(s) is associated with, or characterized by, one or more discovery signal parameters, e.g., a duration of each occasion when a discovery signal is transmitted, periodicity of the occurrence of the occasion of discovery signal(s), starting times of the occasions or of the pattern of the discovery signal(s), etc. One or more of the discovery signal parameters are predefined and/or signaled to the wireless device by a network node.
A wireless device may perform measurement(s) on a serving cell of the wireless device, as well as on neighboring cells, over known signals(s) (e.g., reference signals/sequences or pilot sequences). In particular, for an active cell (e.g., an activated serving cell of the wireless device or an activated neighbor cell), the wireless device performs measurement(s) on the active cell using a reference signal(s) transmitted by the active cell. For a deactivated cell (e.g., a deactivated secondary cell of the wireless device), the wireless device performs measurement(s) on the deactivated cell using a discovery signal transmitted by the deactivated cell. The measurement(s) may be performed on cells on an intra-frequency carrier, inter-frequency carrier(s), as well as on inter-Radio Access Technology (RAT) carriers(s) (depending upon the wireless device capability, e.g., whether it supports that RAT). In a multi-carrier or CA environment, a wireless device may perform measurement(s) on cells on the PCC, as well as on cells on one or more SCCs. Examples of measurements performed in Long Term Evolution (LTE) include cell identification, Reference Signal Received Power (RSRP), and Reference Signal Received Quality (RSRQ). There are other properties that can be derived from a reference signal such as, for example, Quasi Co-Location (QCL), coarse time and frequency synchronization, CSI, Reference Signal Time Difference (RSTD) measurements, fine time and frequency synchronization for demodulation purposes, etc.
The results of performing the measurement(s) by the wireless device are sometimes referred to as radio measurement(s) and are used by the wireless device for one or more radio operational tasks. Examples of such radio operational tasks include reporting the measurements to a node on a network (e.g., a cellular communications system), which in turn may use the measurements for various tasks. For example, in a Radio Resource Control (RRC) connected state, a wireless device reports radio measurements to a serving network node. In response to the reported wireless device measurements, the serving network node makes certain decisions, e.g., it may send a mobility command to the wireless device for the purpose of cell change. Cell change is an example of mobility operations that includes handover, RRC connection reestablishment, RRC connection release with redirection, PCell change in CA, PCC change in PCC, etc. In an IDLE or low activity mode or state, an example of cell change is cell reselection. In another example, a wireless device may itself use radio measurement(s) for performing tasks, e.g., cell selection, cell reselection, etc.
In multicarrier or CA operations, a wireless device is able to receive and/or transmit data to more than one serving cells. In other words, a CA capable wireless device can be configured to operate with more than one serving cell. The carrier of each serving cell is generally called a CC. In other words, the CC is an individual carrier in a multi-carrier system.
As noted above, the term carrier aggregation (CA) is interchangeable with “multi-carrier system,” “multi-cell operation,” “multi-carrier operation,” and “multi-carrier” transmission and/or reception. This means that the CA is used for transmission of signaling and data in the uplink and downlink directions. One of the CCs is the PCC, or simply referred to as the primary carrier or anchor carrier. The remaining CCs are referred to as SCCs, or simply referred to as secondary carriers or supplementary carriers. The serving cell is interchangeably referred to as a PCell or a Primary Serving Cell (PSC). Similarly, the secondary serving cell is interchangeably referred to as a SCell or a Secondary Serving Cell (SSC).
Generally, the primary or anchor CC carries essential wireless device specific signaling. The primary CC (also referred to as PCC or PCell) exists in both uplink and downlink directions in CA. Where there is only a single uplink CC, the PCell is obviously on that CC. The cellular communications system (i.e., network) may assign different primary carriers to different wireless devices operating in the same sector or cell.
A SCell may be configured and de-configured for a wireless device. The configuration procedure is used by a serving radio network node (e.g., an enhanced or evolved Node B (eNB) in LTE) to configure a CA capable wireless device with one or more SCells (i.e., a downlink SCell, an uplink SCell, or both). On the other hand, the de-configuration procedure is used by the radio network node (e.g., an eNB) to de-configure or remove one or more already configured SCells (i.e., a downlink SCell, an uplink SCell, or both). The configuration or de-configuration procedure is also used to change the current multi-carrier configuration, e.g., for increasing or decreasing the number of SCells, or for swapping the existing SCells with other SCells or new ones. The configuration and de-configuration is performed by the eNB in LTE.
A SCell can be activated and deactivated (i.e., turned on and off, respectively). The serving radio network node (e.g., an eNB in LTE) can activate one or more deactivated SCells, or deactivate one or more SCells on corresponding configured secondary carriers. The PCell is always activated. The configured SCells are initially deactivated upon addition and after a cell change, e.g., handover. In LTE, the activation and deactivation commands are sent by an eNB via a Medium Access Control (MAC) Control Element (CE). Notably, deactivation of a SCell saves wireless device battery power.
Measurements may be performed on a SCC with a deactivated SCell. More specifically, a wireless device may perform measurements on a deactivated SCell, or other cells on the SCC with the deactivated SCell. In this case, the measurements are performed by the wireless device on one or more cells on the SCC with the deactivated SCell in accordance with a SCell measurement cycle parameter (i.e., measCycleSCell), which is configured by a protocol layer higher than a physical layer. A measurement cycle may have a periodicity of, for example, 160, 256, 320, or 512 subframes. The maximum time of a measurement within each cycle is currently not restricted by a standard, but in practice the maximum is likely to be up to six subframes in each cycle.
The current standard for LTE (i.e., 3GPP TS 36.133 Rel-10) specifies requirements for interruptions on a PCell when a wireless device performs measurements on a SCC with a deactivated SCell. For example, according to the current standard, when no Discontinuous Reception (DRX) is in use, the wireless device shall be able to identify a new detectable Frequency Division Duplexing (FDD) or Time Division Duplexing (TDD) cell on the SCC within Tidentify_scc, according to the parameter measCycleSCell, where Tidentity_scc=20 measCycleSCell.
Moreover, the measurement period for a deactivated SCell measurements is Tmeasure_scc, according to the parameter measCycleSCell, where Tmeasure_scc=5 measCycleSCell. According to the current standard, a wireless device shall be capable of performing RSRP and RSRQ measurements for eight identified cells on the SCC, and the wireless device physical layer shall be capable of reporting measurements to higher layers (e.g., protocol layer) with the measurement period of Tmeasure_scc.
Depending on the type of information that a cellular communications system provides to a wireless device regarding reference signal(s) that the wireless device is intending to use for performing measurement(s), the wireless device adopts a measurement procedure. Under normal operation of cells, the reference signal(s) that are used for performing measurement(s) are transmitted periodically and also frequently from network node(s). For example, reference signal(s) may be transmitted in every subframe by a base station.
In existing systems, a wireless device behavior for performing measurement(s) on a carrier associated with a deactivated SCell is also determined by the conventional SCell measurement cycle parameter (i.e., measCycleSCell), which again is configured by higher layers of a cellular communications system. Thus, existing measurement procedures use the same SCell measurement cycle parameter to perform measurement(s) on both carriers associated with only activated cells and carriers associated with at least one deactivated cell. This may lead to less than desirable measurements on the deactivated cell(s). Accordingly, a need exists for a measurement procedure for performing measurement(s) on cell(s) that include a deactivated SCell.